When a fluid flows past a solid surface at high speed, turbulence develops at the interface between the fluid and the surface. Within a conduit, the fluid near the walls of the conduit moves more slowly than the fluid near the center. Crosscurrents and countercurrents develop which cause turbulence in the bulk fluid, and the net result of turbulence is a loss of energy and a consequent reduction in velocity of the moving fluid because some of the energy in the fluid velocity is converted to heat. Because of this net loss of fluid velocity, flow reduction by turbulence is often referred to as "fluid drag".
The practical significance of turbulence within a fluid flowing in a conduit can be illustrated in connection with firefighting, where turbulence severely limits equipment performance. In a pumper which has the capacity to send water, for example, 100 feet, a significant fraction of the energy originally imparted to the water is lost as heat because of turbulence. If the fluid drag caused by this turbulence could be reduced, the pumper would be able to deliver the water much further, possibly up to 150-200 ft. with the same energy input.
In 1948, the use of polymers to reduce fluid drag was discovered by B.A. Toms. It was found that the presence of very small amounts of select high molecular weight polymers could reduce turbulence and consequently fluid drag was lowered significantly. For example, the presence of about 100 ppm poly(ethyleneoxide) having a molecular weight of about 5.times.10.sup.6 can reduce turbulence by as much as 50% in water flowing under conditions having a Reynolds Number of about 15,000.
Since the turbulence which is generated at the interface between a moving solid surface and fluid can also cause fluid drag, there is considerable incentive to find methods of reducing the drag which occurs as a vessel passes through a body of water. U.S. Pat. No. 3,303,811, Giles (1967), discloses a method of reducing drag for vessels traveling through water by introducing high molecular weight polymeric additives, such as poly(ethyleneoxide), polyacrylamide and polyvinylpyrrolidone. U.S. Pat. No. 3,720,216, Wartman (1973), describes reducing fluid drag in water under turbulent flow conditions by adding about 50 ppm of finely divided poly(ethyleneoxide) having a molecular weight of about 4.times.10.sup.6, or generally from 10.sup.6 to 10.sup.7.
The problem of drag reduction also has military significance as shown by U.S. Pat. No. 4,186,679, Fabula, et al. (1980), which describes reducing drag on a torpedo by ejecting a water soluble polymer from ports in the nose of the torpedo as it passes through the water. The polymer which is preferred is poly(ethyleneoxide), having a molecular weight of 4.times.10.sup.6 or more. Another polymer disclosed as suitable is polyacrylamide, having a molecular weight of 2.times.10.sup.6. In a similar fashion, U.K. Patent 1,605,138 (1982) discloses using drag reducing agents such as poly(ethyleneoxide) and polyacrylamide to increase propulsion efficiency of a bladed propeller.
The fluid drag problem also exists in materials handling as shown by U.S. Pat. No. 4,637,418, Karl (1987), which describes using as a drag reducing agent a polymer of 2-acrylamido-2-methylpropane sulfonic acid in coal slurried in alcohol. The polymer preferably has a molecular weight above 5.times.10.sup.6.
An interesting method of introducing the drag reducing agent where it is needed is described by U.K. Patent 2,204,047 (1988), which discloses releasing a drag-reducing agent from paint or a coating on a solid surface, such as the hull of a boat, thereby reducing turbulence developed at the interface of liquid flowing over the solid surface. The benefits include both increasing boat speed and decreasing the audio noise along the hull, which obviously has military significance.
A very comprehensive discussion of drag reduction is presented by Kulicke, et al., "Drag Reduction Phenomenon with Special Emphasis on Homogeneous Polymer Solutions", Advances in Polymer Science, Vol. 89 (1989). This article presents a general discussion of the hydrodynamics of drag reduction and the commercial potential for polymeric flow improvers. Poly(ethyleneoxide) and polyacrylamides, as well as other natural and synthetic polymers, are discussed with respect to their ability to effect drag reduction. It is concluded that polymeric additives show pronounced flow phenomena in drag reduction with increasing molecular weight of the polymer.
The necessity of having a polymer with high molecular weight introduces a problem with respect to the stability of the drag reducing agent. High molecular weight polymers tend to be broken down by the turbulence for which they are added to diminish. Consequently, such polymers tend to lose their effectiveness and need to be replaced. For example, in the trans-Alaska pipeline from Prudhoe Bay to Valdez, oil soluble polymers need to be added to the oil in the pipeline approximately every 60 miles in order to maintain reduction of turbulence and energy loss in the pipeline. It is desirable, therefore, to find polymers which are effective drag reducing agents, but remain more stable under turbulent fluid flow conditions.
U.S. Pat. No. 4,490,557, Dawson, et al. (1984), discloses poly(vinylamine) salts which can be made by acid hydrolysis of poly(N-vinylformamide). These poly(vinylamines) have molecular weights ranging from 4,000 to 8.times.10.sup.5. Poly(vinylamines) of still higher molecular weight on the order of 10.sup.6 and greater are disclosed in U.S. Pat. No. 4,843,118, Lai, et al. (1989). These polymers are said to be useful in enhanced oil recovery. Although such polyvinylamines and their hydrochloride salts are well known, it has not heretofore been recognized that they might be useful as drag reducing agents for the various applications discussed above, where fluid drag is a serious problem.